1. Field of the Invention
The present invention relates to a method of and an apparatus for producing a halftone image with an image scanner.
2. Description of the Prior Art
A method of producing a halftone image is disclosed in, for example, Japanese Patent Laying Open Gazette No. 55-6393. A halftone image formed by such a method, however, has a deformed contour and/or a broken line portion. In addition, if the halftone image includes a repetitional pattern, moire may occur due to interference between the repetitional pattern and an array of halftone dots.
As a method to suppress the moire, Japanese Patent Publication Gazette No. 63-16945, for example, discloses a method of previously preparing a plurality of density distribution patterns for originals and a plurality of screen threshold patterns, dividing an unit block of a halftone dot into partial areas, selecting one of the density distribution patterns proper to each partial area, and selecting the screen threshold pattern corresponding to the selected density distribution pattern.
However, in order to obtain an halftone image of high quality by the method disclosed in Japanese Patent Publication Gazette No. 63-16945, the number of screen threshold patterns to be previously prepared must be increased depending on both of the number of partial areas and the number of gradation levels in each partial area. For example, when the number of partial areas is six and the number of gradation levels in each partial area is 16, the number of screen threshold patterns to be prepared is 16.sup.6, sixteen to the sixth power. This means that the method requires a memory having enormous capacity of storing that number of screen threshold patterns. Accordingly, it is difficult to obtain a halftone image of high quality by this method in terms of memory capacity.
In general, the smaller the number of screen threshold patterns becomes, the larger the difference between the screen threshold patterns becomes. Since the screen patterns represents shapes of halftone dots in a reproduced image, if the number of the screen threshold patterns is reduced, the shapes of halftone dots change considerably in response to the change of the density level in an original image; this causes graininess noise in the reproduced image.